Feature Article

Using Side Scan Imagery
To Verify Patch Test Data
Comparing Results of Imagery, Bathymetry
From a Colocated Sensor to Validate Data

AUTHOR:

Harold OrlinskyChief Operations Officer

Judy BraggTechnical Writer
HYPACK Inc.
Middletown, Connecticut

Southbound bathymetric data from the Ohio River tests.

Most shallow-water multibeam sonars on the market allow for the concurrent collection of backscatter data with time-series depths (range and angle) for bottom mapping. The system generates the signal from the same location, providing two distinct data sets but sharing the same mount angle and installation biases. Prior to processing any bathymetry data, this bias—the angular mounting difference between the sensor and the motion unit—must be determined. The United States Army Corps of Engineers (USACE), along with NOAA and other hydrographic groups, have set standards in determining this bias, commonly referred to as a patch test.

The patch test can discriminate the roll, pitch and yaw of the mounting offsets between the multibeam sonar head and the motion reference unit (MRU). The fourth variable calculated in the patch test is the time latency of the positioning system.

These offsets are generally very small: in the magnitude of 1 to 3 degrees for the angular offsets and less than 500 milliseconds for the latency. It would be challenging to measure them in the same manner as the physical placement of the sensors (with a tape measure or total station); therefore, the bathymetry and software are used to accurately calculate these biases.

The Patch Test Survey
The patch test survey consists of three lines. For roll, a line is run in both directions at a constant speed in an area of flat bathymetry. The outer portion of the overlapping swath will show a depth difference, increasing in magnitude with larger roll offsets. In deeper water, the multibeam swath becomes larger, and the error is easily computed.

Pitch and latency tests require lines run over either a slope or a distinct feature. For pitch, the same line is run once in each direction at constant speed. For latency, the same line is run in the same direction, but at different speeds. Since the pitch and latency both look for a horizontal shift, latency should be determined first.

The yaw test requires two parallel lines run over a sloping bottom and spaced at a distance equal to the minimum water depth (top of slope). This provides an overlapping swath, which is used to best align the data.

There is no extra work needed during the survey to use backscatter for patch test calibration. The idea is to use existing data with extra information to perform this analysis. The survey lines (speed, direction and length) are the same as those for the bathymetry patch test.

The sonars themselves have the option to collect the backscatter data, typically with a menu option in the sonar. HYPACK's multibeam HYSWEEP logged the data for four test locations: the Ohio River in Kentucky, the Iowa River in Iowa City, Iowa, and harbors in Philadelphia, Pennsylvania, and New York. Data were collected from 2009 to 2011, with survey depths from 5 meters to approximately 20 meters. In this software, a menu option in the sonar setup enables logging of the backscatter with the bathymetry data. The files log both data sets with identifiers: RMB for raw multibeam in the bathymetry and RSS for raw side scan in the backscatter data.

During data collection, the surveyor observed the data in the waterfall window showing the backscatter data and noted bottom features that could be useful for the backscatter calibration: a feature perpendicular to the trackline to measure latency, pitch and yaw, and a distinct feature at the outer edge of the swath to test for roll. Not all lines will have features, but knowing where they are reduces analysis time. During the patch test, the bathymetry data should be cleaned of excessive noise.

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Harold Orlinsky is chief operations officer at HYPACK Inc. He previously worked as a NOAA Corps officer, serving onboard two vessels, and at the Coast Survey Research and Development Laboratory. He has been in the field of hydrography and surveying for close to 20 years.

Judy Bragg is the technical writer for HYPACK Inc. She has been with the company for 14 years, and is responsible for updating all technical documents and posting software updates on the company's website.

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